This invention relates generally to feedback control for turbine driven high powered alternating current synchronous generator systems.
This invention relates more specifically to generator systems of the above type which are utilized to provide electrical power for loads over relatively long transmission lines where the effective series connected compensating capacitors may introduce torsional oscillation into the mechanical portion of the power generating system. Electrical generators are often driven from power sources such as steam turbines. Multiple rotating masses are usually present on the single shaft thereof. As an example, the rotating mass of one or more of the turbines and the rotating mass of the exciter for the AC generator and the rotating mass of the generator rotor may all be present. It has been found that because of the mechanical properties of the various parts of the rotating system, torsional oscillation will result involving the rotating masses. If these torsional oscillations become large and undamped, they could cause shaft breakage. It has been found in certain applications that such undamped torsional oscillations may exist because of the electrical properties of electrical system supplied by the AC synchronous generator. As an example, when long transmission lines are required for the delivery of energy to a grid network or load, the inductive effect of the long transmission lines may require series capacitive compensation. However, the effect of the compensating capacitors being connected in series circuit relationship with the inductance tends to cause certain resonant frequencies to exist in the electrical network. The electrical effect of these resonant frequencies may be fed by the magnetic coupling between the stator and the rotor of the synchronous generator to the mechanically rotating shaft. If the previously described electrical resonance cooperates with a critical mechanical resonance between the masses of the generating system, reinforced, undamped torsional oscillation will occur which may be of sufficient magnitude to break the shaft. Apparatus and method for solving this problem has been proposed in the past in U.S. Pat. No. 3,662,251, issued May 9, 1972 to O. J. M. Smith, entitled, "Method and System for Measuring Acceleration and Velocity". This latter patent is related to the sensing of oscillations rather than providing a method for directly correcting for the effect of the oscillation. A similar measuring method and apparatus is described in U.S. Pat. No. 3,662,252, issued May 9, 1972 to O. J. M. Smith and entitled, "Tachometer and Method of Obtaining a Signal Indicative of Alternating Shaft Speed". Other U.S. patents propose apparatus for stabilizing low frequency power oscillation by measuring some parameter of a system and operating on the field excitation of the generator for corrective action. Examples of these patents are: U.S. Pat. No. 3,477,014, issued to A. L. Blythe on Nov. 4, 1969 and entitled, "Electrical Control System With Stabilizing Control Means", U.S. Pat. No. 2,981,882, issued Apr. 25, 1961 to M. Rosenblatt, entitled, "Stabilizing Circuit for Dynamoelectric Machines" and U.S. Pat. No. 3,656,048, issued Apr. 11, 1972 to A. W. Hauf, entitled, "Nonlinear Exciter Controller for Power System Damping". Patents which are of interest for showing field control in electrical apparatus are: U.S. Pat. No. 2,854,617, issued Sept. 30, 1958 to L. J. Johnson and entitled, "Frequency Control Apparatus for Alternators" and U.S. Pat. No. 3,474,323, issued Oct. 21, 1969 to L. A. Kilgore et al., entitled "Electrical Control Systems With Stabilizing Control Means". Still other patents which may be of interest generally in this area are: U.S. Pat. No, 3,119,934, issued Jan. 28, 1964 to R. H. Lee, entitled, "Generator Control Means" and U.S. Pat. No. 3,167,702, issued Jan. 26, 1965 to A. Schonung et al., entitled, "Vibration Damping Arrangement for Rotary Field System of Excitation Regulation Synchronous Machines". Another patent which is related to frequency control by mechanical means is U.S. Pat. No. 2,767,367, issued Oct. 16, 1956 to L. Black and entitled, "Generator Frequency Control by Electric Breaking". All of the aforementioned patents are background patents for a U.S. Pat. No. 3,999,115, issued Dec. 21, 1976 to South et al. and entitled, "Dynamic Stabilizer for Synchronous Machines Having Torsional Oscillations and Method". It is submitted that the latter patent represented a significant improvement in the state of the art for compensating for the effect of mechanical torsional oscillation as it is affected by electrical power line resonance, where the compensation takes place in the electrical system per se. However, even the latter patent had certain disadvantages. One disadvantage lies in the fact that the compensating apparatus of the latter patent is essentially series connected in the transmission system to be compensated. Furthermore, the apparatus of the latter mentioned patent requires a frequency conversion. The frequency conversion is related to the frequency of the line resonance rather than the frequency of the torsional oscillation resonance. The former disadvantage means that the compensating device must continuously pass the total output current of the generator. Furthermore, in order to take the device out of the system, it must be short circuited. The latter disadvantage means that a frequency conversion must be made. It would be advantageous therefore, if apparatus could be found in which the compensating device was essentially a parallel connected device rather than a series connected device and in which the actual torsional oscillation frequency could be used as a modulating signal rather than the frequency of the resonance of the line.